Quite often, the diagnostic tools and techniques at the disposal of your mobile veterinary service are all that are needed to solve the mystery. Some horse owners have been through this routine a time or two: Your vet will ask to see your horse move on a straight line and on a circle, then on a hard surface and a soft one. He or she will ask when you're most likely to notice the gait abnormality: At a trot or at a canter? At the beginning of a workout or at the end? Can you think of any incident that might have caused an injury such as a particularly hard gallop or getting a hoof caught in the fence?

The veterinarian might begin by palpating the leg, gently squeezing the tendons and ligaments or probing the texture of a knee or ankle with his or her fingers, looking for subtle signs of inflammation, and/or he or she might use hoof testers to put pressure on the sole of Boomer's foot, looking for a sensitive spot. He or she might employ flexion tests, where the horse's leg is held in a position of maximum joint flexion for a minute or two, then he's asked to trot out briskly, a technique that can highlight some types of lameness.

If these methods don't reveal the source of the complaint, most veterinarians will suggest they "block" the leg with injections of a local anesthetic, starting down at the ground and gradually working up the limb. After each injection, the horse is walked and trotted until the anesthetic has numbed the nerves in the problem area and the horse moves soundly. Very often, this time-honored technique helps zero in on the location of a lameness, but it won't tell you exactly what's going on in there. For that, your veterinarian might have to get high-tech help.

On-Farm Imaging

The 21st Century has brought us a dizzying number of ways to peek inside a horse's limbs, each with its own strengths and weaknesses. But the one with which we're most familiar is the tried-and-true radiograph or X ray‚ a staple of almost every mobile equine veterinary practice the world over.

Radiographs capture ionizing energy (radiation) produced by a small machine on a plate of silver embedded in emulsion. The rays pass through soft tissue, but are stopped by bone, creating an image in which bony structures show up as white, while muscle, fatty tissue, ligaments, hooves, and tendons are invisible or show up as mere shadows.

Portable radiograph machines have improved dramatically over the years, both in terms of resolution and contrast. They can create images in fractions of a second rather than minutes, using far less intense radiation than the machines of a few decades ago (although precautions still need to be taken by humans to avoid unnecessary exposure). All of this means your veterinarian can often achieve sharp diagnostic images even with an X ray machine designed for on-farm use--but the results depend on the precise positioning of the plate behind the limb, on the cooperation of the horse (since any movement will blur the image), and on the steadiness of the camera and the hands holding the plate. With most X rays, you also have to wait until your veterinarian can develop the film back at the clinic before you know what they reveal.

Another imaging technique that has become portable is ultrasound. Originally used largely for monitoring heat cycles and pregnancies in broodmares, ultrasound machines have become increasingly popular for the diagnosis of lameness. Where radiographs reveal bone, ultrasound allows us to look into the soft tissues of the horse, making it the modality of choice for suspected tendon and ligament problems as well as muscle abnormalities.

Ultrasound uses harmless, high-frequency sound waves that bounce back at different intensities depending on the density of the tissues they encounter. These waves require no special shielding or precautions. It is also an instant gratification technique, displaying results on a computer screen in real time as your veterinarian moves the probe around the suspected problem area on your horse's leg. But perhaps even more so than with radiography, interpreting a black-and-white ultrasound image takes experience and practice, and correct placement of the probe is essential for good results.

With these tools at your veterinarian's disposal, many lamenesses can be assessed on-farm. But there are still some limitations. X rays, for example, can sometimes fail to reveal crucial details like bone chips or fractures because they're essentially two-dimensional images of three-dimensional objects, and thus are dependent on the angles and views taken. Some portable ultrasound units might not have the power, resolution, or emit the right sound wave frequencies to penetrate the tissue your veterinarian needs to examine.

What do you do if, despite your veterinarian's efforts, you still have a mystery lameness?

Going for High Tech

When ordinary methods fail, it might be time go to the nearest veterinary college or private equine hospital to take advantage of some high-tech diagnostics.

The good news is several modalities now available can explain the inexplicable as never before. Depending on your veterinarian's recommendation and local availability (and to some extent, your budget), your horse can be examined by way of nuclear scintigraphy, computed tomography, and/or magnetic resonance imaging, to name a few.

Most of these imaging techniques have been borrowed from human medicine, often with a time lag that represents the innovations and changes necessary to adapt the machines to accommodate a 1,000-pound animal that doesn't lie still when he's told. Still, you might find yourself feeling like an outtake from "ER" as you observe your horse's hock coming up in fluorescent color cross-section on a computer screen. But the answers these imaging techniques might provide can assist tremendously, not only with diagnosis, but also with prognosis and treatment. And as veterinarians become more experienced and skilled in using these devices, many of these modalities have lent themselves to new possibilities for the future.

Let's have a look at six different methods of diagnostic imaging, one by one:

  • Digital radiography;
  • Digital ultrasound;
  • Fluoroscopy;
  • Nuclear scintigraphy;
  • Computed tomography; and
  • Magnetic resonance imaging.

Going Digital

Although the technology behind X rays is more than 100 years old, that doesn't mean there isn't room for improvement. One of the most exciting advances is digital radiography, which allows a radiology technician to adjust the resolution and contrast of the image for a sharper, clearer picture of the bone.

Mark Martinelli, DVM, MS, PhD, Dipl. ACVS, specializes in lameness and diagnostic imaging at his California Equine Orthopedics in southern California. Of digital radiography, he says, "There are two formats: First, there's computed radiography, which uses a plate with a phosphor screen instead of film. The plate is inserted in a machine that reads the information off it and displays it on a computer screen, so there's no need for a darkroom and the images are available much faster than with traditional plates.

"Then there's DR, for direct--or digital--radiography," Martinelli continues. "This variation uses an 8"x10" plate attached by a cord to a computer. The image goes directly to the computer in four to five seconds, and the plates are re-usable. At the moment, DR is expensive, but the units are coming down in price, and wireless machines are in the works. I think they're going to revolutionize private ambulatory practices, not only because you get high-quality instant results, but because you can use DR to radiograph regions previously difficult to radiograph in the field, such as the neck or stifle. Because DR allows computer enhancement of the images in regions where there's a lot of soft tissue to penetrate, these areas can be more effectively imaged either in the clinic or even in the field with a portable X ray unit."

William Hornof, DVM, MS, Dipl. ACVR, former chief of radiology at the University of California, Davis (having recently left for private practice in the imaging industry), notes, "One of the great advantages of digital radiography over conventional X rays is that you don't have to worry about over- or under-exposure. You just dial the image up or down; it corrects for any exposure problems."

At the moment, digital and computed radiology tends to be found in hospital or clinic settings, although as Martinelli predicted, they are gradually making their way into the field.

Shipping in to a clinic might also be necessary if you need to expose your horse to a larger, more powerful X ray machine to help diagnose a skeletal injury, such as one in a shoulder or pelvis, which is beyond the means of a hand-held unit.

Ultrasound machines are also going digital. "You can do (an ultrasound) off a little laptop now," says Martinelli. "You attach the probe directly to the laptop and get images on the screen. And the computer can save those images as you go and form a 3-D composite--a 'cube' that can be viewed in different planes and cross-sections."

This kind of imaging can often reveal tiny details, such as a frayed section of a flexor tendon, that might previously have been missed.

"One of the neat things about the laptop unit is that the veterinarian can save the images, then go back to the clinic and view the whole examination again as if it were real-time, like a running video. That's a brand new concept in soft-tissue diagnostics," Martinelli adds.

"The gap is narrowing between the top- and bottom-end ultrasound machines," says Hornof. "As the technology has matured, ultrasound has become the diagnostic of choice for tendon and ligament problems. It's very, very good for any tendon or ligament close to the surface, though it can't see through the hoof capsule."

If there's a snag in all of these digital innovations, it's in the realm of data storage. Martinelli says, "We're generating so much data now that archiving is beginning to be an issue. The files can get very large, especially digital radiographs, which can be huge. Fortunately, there are companies specializing in storing this kind of data in human medicine that are now turning their attention to the veterinary field and giving us options. We want to be able to store patient information and the diagnostic images in a single file, which is a little different from how it's approached in human medicine. But there's software in development now."

We're Going Live

Imagine an X ray machine that shows you not a static image, but a live and continuous view of the bony innards of your horse. That's the fluoroscope, a hand-held scanner shaped like a large C, which displays its findings on a TV monitor as it's manipulated around your horse's leg.

Fluoroscopy generally lacks the sharp detail achievable by high-end X ray units, but it makes up for that by providing instantaneous images that aren't affected by slight movements by the horse. Furthermore, using a fluoroscope is fairly cost-effective. At Kleider Veterinary Services in Langley, British Columbia, Nick Kleider, DVM, finds that scanning an entire leg with the fluoroscope often works out to approximately what it would cost to do static X rays of a single joint. That's a huge bonus if you're working with a mystery lameness and haven't been able to zero in on one location.

"I use the fluoroscope routinely in lameness diagnostics," says Kleider. "I've had the unit for about a decade now, and I would find it very difficult to run my practice without it! What you lose in resolution is made up for by the panoramic view it provides, of any part of the horse you can fit in between the arms of the fluoroscope, right up to the stifle."

Often Kleider uses the fluoroscope on mystery lamenesses before, or instead of, blocking the limb. He finds it a less invasive way to narrow down the location of the problem. Then, if the fluoroscopic image isn't sufficiently detailed to tell the whole story, he employs conventional radiographs to bring the lameness into sharper focus.

"It's great for hind-end lamenesses in particular--especially subtle ones with no discernible swelling or heat," says Kleider. "The 360-degree view can help you view each individual joint in the hock, for example. Then you can identify which angle you need to radiograph to make a definitive diagnosis. Or you can do a motion study by taking the leg and flexing it while you're scanning--that can highlight articular fractures, like in the proximal sesamoids, for example.

"About 50% of the time (the fluoroscope) can show you the problem and save the owner a lot of time and money," he adds.

Kleider found there was something of a learning curve to interpreting fluoroscopic images at first, since they're the reverse of X rays. "White is black and black is white," he says. "But it's so quick to use--sometimes you can reveal a problem in three seconds."

That fluoroscopy hasn't become a more mainstream part of equine veterinary practice surprises Kleider a little, considering the prominent role it plays in his diagnostic strategy. "It's very under-rated!" he says. "I guess that it never really got popular because the equipment is easily damaged. It was originally advertised as portable, but when I took it to the track I found it was too fragile--I was always replacing parts. Now I just keep it at the clinic and scan on an outpatient basis. And I do sedate the horse because it's such an expensive piece of equipment."

Concerns about radiation exposure might have dissuaded some practitioners from employing fluoroscopes, but according to Kleider, the amount of radiation emitted by a fluoroscope is very low compared to conventional X rays. Protection against radiation exposure is still necessary, but he has yet to encounter any difficulties in more than 10 years of use.

One thing the fluoroscope isn't good at is scanning the hoof. "Usually the resolution you get isn't great, and it's difficult to maneuver the arms to get a good image unless you pick up the foot. But it's a great monitoring tool during surgery on bone chips--you can get an instant view of the joint and see whether you've gotten all of the fragments before you close. And I find it's wonderful for repairing fractures to see where I should position the screws. It's so much faster than developing X rays."

With the addition of some injectable contrast dye, fluoroscopy can also be used to check that a temporary nerve block has gone where it's supposed to go. "You get an instant yes or no on the needle placement, which can be a very good control," Kleider says.

"And it's very client-friendly, because it's digital imaging," he adds. "You can freeze-frame the images--you lose a little resolution, like a videotape when it's put on pause, but it's still good enough to see in most cases. And you can save the whole scan to a disc. Clients love it because they can get a copy and play it on their own computers."

Going Nuclear

Nuclear medicine, once the stuff of science fiction, now is almost routine. Scintigraphy, commonly known as a bone scan, has become one of the most useful ways of identifying the location of a mystery lameness, versatile enough to examine bone or soft tissues (or even the vascular system) depending on the method and the way the technique is used.

"Nuclear scintigraphy is great when you need a screening process, because it's pretty easy to image the entire horse," Martinelli says. "If you don't know which area to focus on, or if there are multiple problem areas, the bone scan will help you identify that.

"In the past, some causes of lameness have been very difficult to diagnose--injuries to the attachment of the collateral ligaments in the hock or stifle, for example, or the deep digital flexor insertion in the foot," he continues. "Scintigraphy makes it easy to see these regions and it's far better than any other imaging technique."

To perform a bone scan, the horse must first be injected with a few ccs of a radioactive isotope labeled with a bone-seeking agent, which is then distributed throughout the body and tends to collect anywhere there's active metabolism, such as growth plates or areas with inflammation or bone remodeling. A gamma camera mounted on a robotic arm, which allows it to move and pivot, scans the mildly sedated animal and displays the denser concentrations of radiation as "hot spots" on a computer screen. The vascular system can be examined immediately after injection, and bones and their ligament attachments a couple of hours later as the isotope migrates through the tissues. Eventually, it ends up in the bladder and is excreted in the urine, leaving no lasting ill effects; it is completely eliminated from the horse's system within 24 hours.

Although scintigraphy is quite safe, the technicians handling the horse do take precautions to limit their exposure to the isotope, wearing gloves and using special disposal techniques for the bedding used for the treated horse.

Howard Dobson, DVM, who oversees the nuclear scintigraphy process at the Ontario Veterinary College at the University of Guelph in Ontario, explains, "Different carrier molecules bind to the active metabolizing tissues. You can do bone scans with it, lung scans, examine kidney function, or look for infection by labeling the white cells. The thing about it is, it is very good at telling you where the problem is, but less useful for telling you what it is."

As nuclear scintigraphy technology continues to improve, that's changing. Even now, scintigraphy is an excellent diagnostic method for differentiating some types of problems inside the hoof. Martinelli explains, "If you have a lameness that blocks out in the foot, but you can't tell which structures are involved in an X ray, a bone scan can tell you whether it's the navicular bone, the coffin bone, or the insertion of the deep digital flexor. It can be that accurate if it's viewed by a clinician with practice and experience."

Martinelli finds that nuclear scintigraphy is often the best starting point for diagnosing a mystery lameness. And with an increasing number of scintigraphy units in place at veterinary hospitals and private clinics, this modality is rapidly moving into the mainstream.


Once known as a CAT scan (for computer-assisted tomography), computed tomography is more commonly called CT these days, for computed tomography. Either way, it's another variation on X ray technology that takes its capabilities into the previously unattainable. In a CT scan, a rotating, focused X ray beam takes "sliced" images of the anesthetized patient as he is advanced through a gantry (the frame housing the unit). As Hornof explains, "It allows exquisite separation of the tissues, like a loaf of bread sliced up. So unlike a conventional X ray where everything is superimposed, a CT can reveal cross-sections one to 10 millimeters thick."

One of the major advantages of CT over other types of diagnostic imaging is the superior resolution. With both magnification and three-dimensional capabilities, lesions too small or well-buried to detect with conventional radiography often jump out on the CT screen.

"It's an excellent way to look inside the hoof capsule," says Hornof, "especially with the addition of some injected contrast dye. The dye 'lights up' the interior structures and shows up as a bright spot on the computer screen, which can generate a new image every second."

He adds, "CT is also most useful for fractures. In cross-section, you can see all the fracture lines and plan where you'll need to insert screws or plates to make the repair."

Most veterinary CT scanners can safely image equine limbs up to the knees and hocks, or do the head and the first four or five inches of the neck. (Hornof notes that the CT scanner at UC Davis is often used to assess cases of chronic sinusitis and dental issues.)

Like other kinds of radiography, computed tomography is particularly good for examining hard tissue (skeletal) problems, and less skilled at soft tissue problems. For a more complete view of the limb's interior structures, many veterinarians are excited about the possibilities of magnetic resonance imaging (MRI), which is just starting to be economically feasible for equine practice.

It's So Magnetic

Equest Diagnostic Imaging in San Marcos, Calif., is the new home of the first "standing" MRI unit for horses in the United States, and Martinelli couldn't be more pleased. "There are a few university vet hospitals which have (non-standing) MRI," he says, "but they require general anesthesia. Our new MRI will allow us to scan horses standing, up to the knee and the hock. No units can currently scan any higher than that."

MRI uses a powerful magnetic field instead of radiation to create images, so no lead aprons are required. The black-and-white images that appear in sequence on the computer screen take only a couple of minutes each to generate, but it might take many "slices" in a 1 1/2-hour session to get a full diagnostic picture. Differences in the signal intensity highlight any abnormalities, especially when compared to an MRI of the opposite limb (presumably normal). Like a CT scan, MRI provides an infinite number of possibilities with regard to manipulating the images--they can be viewed dorsally (from the top of the horse), transversely (horizontal cross-section), sagittally (vertical cross-section), or in any other plane you desire.

What's most remarkable about MRI is the way it can expose all of the internal structures simultaneously. Says Martinelli, "All of the tissues can be revealed at once--tendons, ligaments, bone, everything. So instead of seeing just part of the anatomy, you're getting the whole picture. Because of this, there is no question that MRI is the best imaging technique we have for foot problems. And it can also highlight bone chips or fractures that other techniques might miss. We've done quite a few MRIs of the foot and fetlock in the racehorse already."

Because MRI offers such a complete picture of the interior of the horse's limbs, Martinelli believes it will become more popular than CT as a diagnostic technique. But Hornof sees room for both modalities. "MRI is a buzz right now," he says, "and it's really good for soft tissue, but CT is really good for bone. I predict they're going to end up being complementary. When CT first hit the market 30 years ago, everyone said it would replace radiographs. It hasn't."

Clearly, there's room in the diagnostic world for all of these modalities and more, which can only be a good thing for owners and veterinarians seeking to stamp out mystery lamenesses.

Radiography (X rays)
Magnetic resonance imaging (MRI)
CT scan
Computed tomography (CT scan)

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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